Heretofore, surface-treated metal sheets such as galvanized steel sheets have widely been used as structural members for automotive bodies, etc. In welding such metal sheets with a laser beam, a gas is produced from the metal of the surface-treated layers while the surface-treated layers and the metal sheets are being melted. It is well known in the art that part of the melted metal sheets is blown away to cause sputtering or internal defects under the pressure of the metal gas emitted from the surface-treated layers, thereby tending to cause a welding failure. It is also known in the art to overcome the above shortcoming by forming a gap as a space between the surface-treated metal sheets to allow the metal gas to diffuse into the gap for thereby welding the metal sheets without fail. There have been proposed a variety of processes for forming such a gap between the metal sheets. For example, Japanese Patent No. 3115456 discloses a process of forming a gap by keeping free one end of a metal sheet to which a laser beam is applied, based on the difference between volumetric shrinkages of stacked metal sheets caused when the melted metal sheets are solidified and cooled in a laser forming process.